Oh my goodness! That doesn't sound good at all. I've got no experience in the saltwater world, but I did promote your post to alert the other folks on the forum to help you out if they know anything that will help you. All I can say is that if your triggerfish keeps pestering your horseshoe crab, then I would remove the triggerfish temporarily until someone can help you out.
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Phishie

"An ounce of prevention is worth a pound of cure."